Threshold ECDSA

Overview

We present a minimal example canister smart contract for showcasing the threshold ECDSA API.

The example canister is a signing oracle that creates ECDSA signatures with keys derived from an input string.

More specifically:

The sample canister receives a request that provides a message.
The sample canister hashes the message and uses the key derivation string for the derivation path.
The sample canister uses the above to request a signature from the threshold ECDSA subnet (the threshold ECDSA is a subnet specializing in generating threshold ECDSA signatures).

This tutorial gives a complete overview of the development, starting with downloading dfx, up to the deployment and trying out the code on the mainnet.

This walkthrough focuses on the version of the sample canister code written in Motoko programming language, but no specific knowledge of Motoko is needed to follow along. There is also a Rust version available in the same repo and follows the same commands for deploying.

Prerequisites

Download and install the BIG SDK if you do not already have it.
Clone the example dapp project: git clone https://github.com/dfinity/examples

Getting started

Sample code for threshold-ecdsa is provided in the examples repository, under either /motoko or /rust sub-directories. It requires at least BIG SDK version 0.11.0 for local development.

Deploy and test the canister locally

This tutorial will use the Rust version of the canister:

cd examples/rust/threshold-ecdsa
dfx start --background
npm install
dfx deploy

What this does

dfx start --background starts a local instance of the BIG via the BIG SDK
dfx deploy deploys the code in the user's directory as a canister on the local version of the BIG

If successful, you should see something like this:

Deployed canisters.
URLs:
  Backend canister via Candid interface:
    ecdsa_example_motoko: http://127.0.0.1:4943/?canisterId=t6rzw-2iaaa-aaaaa-aaama-cai&id=st75y-vaaaa-aaaaa-aaalq-cai

If you open the URL in a web browser, you will see a web UI that shows the public methods the canister exposes. Since the canister exposes public_key and sign methods, those are rendered in the web UI.

Deploying the canister on the mainnet

To deploy this canister the mainnet, one needs to do two things:

Acquire cycles (equivalent of "gas" in other blockchains). This is necessary for all canisters.
Update the sample source code to have the right key ID. This is unique to this canister.

Acquire cycles to deploy

Deploying to the BigFile requires cycles. You can get free cycles from the cycles faucet.

Update source code with the right key ID

To deploy the sample code, the canister needs the right key ID for the right environment. Specifically, one needs to replace the value of the key_id in the src/ecdsa_example_rust/main.mo file of the sample code. Before deploying to mainnet, one should modify the code to use the right name of the key_id.

There are three options:

dfx_test_key: a default key ID that is used in deploying to a local version of BIG (via BIG SDK).
test_key_1: a master test key ID that is used in mainnet.
key_1: a master production key ID that is used in mainnet.

For example, the default code in src/ecdsa_example_motoko/main.mo includes the following lines and can be deployed locally:

The following example is two code snippets that are part of a larger code file. These snippets may return an error if run on their own.

let { public_key } = await ic.ecdsa_public_key({
  canister_id = null;
  derivation_path = [ caller ];
  key_id = { curve = #secp256k1; name = "dfx_test_key" };
});

let { signature } = await ic.sign_with_ecdsa({
  message_hash;
  derivation_path = [ caller ];
  key_id = { curve = #secp256k1; name = "dfx_test_key" };
});

To deploy to BIG mainnet, one needs to replace the value in key_id fields with the values "dfx_test_key" to instead have either "test_key_1" or "key_1" depending on the desired intent.

Deploy to the mainnet via BIG SDK

To deploy via the mainnet, run the following commands:

npm install
dfx deploy --network ic

If successful, you should see something like this:

Deployed canisters.
URLs:
  Backend canister via Candid interface:
    ecdsa_example_motoko: https://a3gq9-oaaaa-aaaab-qaa4q-cai.raw.icp0.io/?id=736w4-cyaaa-aaaal-qb3wq-cai

In the example above, ecdsa_example_motoko has the URL https://a3gq9-oaaaa-aaaab-qaa4q-cai.raw.icp0.io/?id=736w4-cyaaa-aaaal-qb3wq-cai and serves up the Candid web UI for this particular canister deployed on mainnet.

Obtaining public keys

Using the Candid Web UI

If you deployed your canister locally or to the mainnet, you should have a URL to the Candid web UI where you can access the public methods. We can call the public-key method.

In the example below, the method returns 03c22bef676644dba524d4a24132ea8463221a55540a27fc86d690fda8e688e31a as the public key.

{
  "Ok":
  {
    "public_key_hex": "03c22bef676644dba524d4a24132ea8463221a55540a27fc86d690fda8e688e31a"
  }  
}

Code walkthrough

Open the file main.mo, which will show the following Motoko code that demonstrates how to obtain an ECDSA public key.

  //declare "ic" to be the management canister, which is evoked by `actor("aaaaa-aa")`. This is how we will obtain an ECDSA public key 
  let ic : BIG = actor("aaaaa-aa");

  public shared (msg) func public_key() : async { #Ok : { public_key: Blob }; #Err : Text } {
    let caller = Principal.toBlob(msg.caller);
    
    try {

      //request the management canister to compute an ECDSA public key
      let { public_key } = await ic.ecdsa_public_key({

          //When `null`, it defaults to getting the public key of the canister that makes this call
          canister_id = null;
          derivation_path = [ caller ];
          //this code uses the mainnet test key
          key_id = { curve = #secp256k1; name = "test_key_1" };
      });
      
      #Ok({ public_key })
    
    } catch (err) {
    
      #Err(Error.message(err))
    
    }

  };

In the code above, the canister calls the ecdsa_public_key method of the BIG management canister (aaaaa-aa).

The BIG management canister is just a facade; it does not exist as a canister (with isolated state, Wasm code, etc.). It is an ergonomic way for canisters to call the system API of the BIG (as if it were a single canister). In the code below, we use the management canister to create an ECDSA public key. let ic : BIG = actor("aaaaa-aa") declares the BIG management canister in the code above.

Cube root public key

For obtaining the canister's root public key, the derivation path in the API can be simply left empty.

Key derivation

For obtaining a canister's public key below its root key in the BIP-32 key derivation hierarchy, a derivation path needs to be specified. As explained in the general documentation, each element in the array of the derivation path is either a 32-bit integer encoded as 4 bytes in big endian or a byte array of arbitrary length. The element is used to derive the key in the corresponding level at the derivation hierarchy.
In the example code above, we use the bytes extracted from the msg.caller principal in the derivation_path, so that different callers of public_key() method of our canister will be able to get their own public keys.

Signing

Computing threshold ECDSA signatures is the core functionality of this feature. Canisters do not hold ECDSA keys themselves, but keys are derived from a master key held by dedicated subnets. A canister can request the computation of a signature through the management canister API. The request is then routed to a subnet holding the specified key and the subnet computes the requested signature using threshold cryptography. Thereby, it derives the canister root key or a key obtained through further derivation, as part of the signature protocol, from a shared secret and the requesting canister's principal identifier. Thus, a canister can only request signatures to be created for its canister root key or a key derived from it. This means, that canisters "control" their private ECDSA keys in that they decide when signatures are to be created with them, but don't hold a private key themselves.

  public shared (msg) func sign(message_hash: Blob) : async { #Ok : { signature: Blob };  #Err : Text } {
    assert(message_hash.size() == 32);
    let caller = Principal.toBlob(msg.caller);
    try {
      Cycles.add(10_000_000_000);
      let { signature } = await ic.sign_with_ecdsa({
          message_hash;
          derivation_path = [ caller ];
          key_id = { curve = #secp256k1; name = "dfx_test_key" };
      });
      #Ok({ signature })
    } catch (err) {
      #Err(Error.message(err))
    }
  };

Signature verification

For completeness of the example, we show that the created signatures can be verified with the public key corresponding to the same canister and derivation path.

The following shows how this verification can be done in Javascript, with the secp256k1 npm package:

let { ecdsaVerify } = require("secp256k1")

let public_key = ... // Uint8Array type, the result of calling the above canister "public_key" function.
let hash = ...       // 32-byte Uint8Array representing a binary hash (e.g. sha256).
let signature = ...  // Uint8Array type, the result of calling the above canister "sign" function on `hash`.

let verified = ecdsaVerify(signature, hash, public_key)

The call to ecdsaVerify function should always return true.

Similar verifications can be done in many other languages with the help of cryptographic libraries that support the secp256k1 curve.

Conclusion

In this walkthrough, we deployed a sample smart contract that:

Signed with private ECDSA keys even though canisters do not hold ECDSA keys themselves.
Requested a public key.
Performed signature verification.

Threshold ECDSA

Overview​

Prerequisites​

Getting started​

Deploy and test the canister locally​

What this does​

Deploying the canister on the mainnet​

Acquire cycles to deploy​

Update source code with the right key ID​

Deploy to the mainnet via BIG SDK​

Obtaining public keys​

Using the Candid Web UI​

Code walkthrough​

Cube root public key​

Key derivation​

Signing​

Signature verification​

Conclusion​